– how can we determine the age of planets in the solar system?
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When lava cools, the rocks finally crystallize into typical rock minerals.
Some of the minerals include atoms that may be radioactive, which will then decay over time.
When you find a mineral that has lead atoms where uranium should be, you can infer that the lead used to be uranium and has since decayed over billions of years.
Since we know the decay rate, we can date these rocks by how much uranium has decayed.
The oldest rocks we find on Earth are 4.3 billion years old, suggesting that the Earth was completely liquid before that time.
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